[GHC] #13272: DeriveAnyClass regression involving a rigid type variable

[GHC]

#13272: DeriveAnyClass regression involving a rigid type variable
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:  Generics
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by RyanGlScott:

@@ -7,0 +7,6 @@
+ {-# LANGUAGE DefaultSignatures #-}
+ {-# LANGUAGE DeriveAnyClass #-}
+ {-# LANGUAGE DeriveGeneric #-}
+ {-# LANGUAGE FlexibleContexts #-}
+ {-# LANGUAGE ScopedTypeVariables #-}
+ {-# LANGUAGE TypeFamilies #-}

New description:

 I was writing up an example to show off how `DeriveAnyClass` has improved
 since 639e702b6129f501c539b158b982ed8489e3d09c, and wouldn't you know it,
 the example doesn't actually compile anymore post-
 639e702b6129f501c539b158b982ed8489e3d09c. Oopsie.

 {{{#!hs
 {-# LANGUAGE DefaultSignatures #-}
 {-# LANGUAGE DeriveAnyClass #-}
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 module TypeName where

 import GHC.Generics

 class TypeName a where
   typeName         :: forall proxy.
                       proxy a -> String
   default typeName :: forall proxy d f.
                       (Generic a, Rep a ~ D1 d f, Datatype d)
                    => proxy a -> String
   typeName _ = gtypeName $ from (undefined :: a)

 gtypeName :: Datatype d => D1 d f p -> String
 gtypeName = datatypeName

 data T a = MkT a
   deriving (Generic, TypeName)
 }}}

 This compiles before that commit. After it, however, it fails with the
 error:

 {{{
 [1 of 1] Compiling TypeName         ( Bug.hs, interpreted )

 Bug.hs:23:22: error:
     • Couldn't match type ‘f’
                      with ‘C1
                              ('MetaCons "MkT" 'PrefixI 'False)
                              (S1
                                 ('MetaSel
                                    'Nothing 'NoSourceUnpackedness
 'NoSourceStrictness 'DecidedLazy)
                                 (Rec0 a))’
         arising from the 'deriving' clause of a data type declaration
       ‘f’ is a rigid type variable bound by
         the deriving clause for ‘TypeName (T a)’ at Bug.hs:14:38
     • When deriving the instance for (TypeName (T a))
    |
 23 |   deriving (Generic, TypeName)
    |                      ^^^^^^^^
 }}}

 I'm not sure why it's complaining only about `f` and not, say, `d`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13272#comment:2>
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